1. Field of Invention
The present invention relates to an initial synchronization method and a receiver for initial synchronization in a DS-CDMA (Direct Sequence-Code Division Multiple Access) inter base station asynchronous cellular system.
2. Description of Related Art
CDMA cellular systems based on code division multiple access (CDMA) using a direct sequence (DS)-type spread spectrum (SS) greatly increases the channel capacity. These systems are receiving much attention in the recent work on ground mobile communication. In general, the frequency utilization efficiency of a CDMA system is inferior to that of other multiple access systems (FDMA, TDMA) because of mutual interference with other stations. However, the cellular system is robust against interference since the spatial frequency re-utilization efficiency (repetition rate of cells having the same frequency) contributes to the efficiency of the overall frequency utilization. Hence, a CDMA cellular system having a high cell repetition rate is assured to be an effective system in the future.
Generally, cellular systems require two kinds of cell searches, namely, an initial cell search by which an initial cell to be connected to a mobile station is identified, and a neighboring cell search by which neighboring cells are searched for the hand-over. Notably in DS-CDMA cellular systems, each of the cells uses the same frequency. Hence, the initial synchronization needs to reduce the timing error between the spread code of the received signal and the spread code replica generated by the receiver to within 1/2 chip period while the simultaneous cell search.
DS-CDMA cellular systems can be categorized into two classes, namely, inter base station synchronous systems in which temporal synchronization is performed strictly among all base stations, and inter base station asynchronous systems in which temporal synchronization is not performed. Inter base station synchronous systems achieve inter base station synchronization using other systems such as the GPS (Global Positioning System). Since each of the base stations uses the same long code by giving it a delay which differs from one base station to another, it suffices to synchronize only the timing of the long code during the initial search. In addition, the neighboring cell search for the hand-over can be performed at a higher speed since the mobile station receives the code delay information of the neighboring base stations directly from the given base station with which the mobile station is at a given time communicating.
On the other hand, in inter base station asynchronous systems, each of the base stations uses a different spread code in order to identify the base stations. The mobile station needs to identify the spread codes in performing the initial cell search. When searching neighboring cells for the hand-over, the number of codes to be searched can be limited by obtaining the spread codes of the neighboring base stations from the current base station to which the mobile station belongs. However, in comparison with inter base station synchronous system, the search time is longer. Indeed when a long code is used for the spread code, the amount of time for the cell search becomes enormous. However, this inter base station asynchronous system has an advantage in that other systems such as GPS are not needed.
A cell search system capable of solving these problems inherent in the inter base station asynchronous systems and performing initial synchronization at a high speed is being proposed by Kenichi Higuchi, Mamoru Sawahashi, and Fumiyuki Adachi, in "Two-step high speed long code initial synchronization method in the DS-CDMA inter base station asynchronous system" (Shingakugihou CS-96-19, RCS96-12 (1996-05)). According to this initial synchronization method, a spread code sequence synthesized from a long code corresponding to the cells and a short code corresponding to a communication channel is used to doubly spread the data. A second short code common to all cells is assigned to the control channel in order to perform the initial synchronization in two steps.
More specifically only the short code is used and the long spread code is masked for the control channel transmitted from the base station of every cell. On the mobile station side, at the first stage, a matched filter de-spreads the received signal by using the short code and detects the timing of the long code. At the second stage, a correlator identifies the long code corresponding to a cell by using a spread code sequence synthesized from the long code corresponding to the cell and the specific short code.
FIG. 1 illustrates a configuration of cells. In FIG. 1, the numeral 61 represents the mobile station. Each of the cells #1 through #n has one of the base stations BS.sub.1, BS.sub.2, . . . , BSn, respectively. Each base station sends a signal to the mobile station 61 by using symbols which are doubly spread by the long codes #1, #2, . . . , #n and the short codes #0-#s identifying each channel. Here, the short codes #0-#s are common to each of the cells. In addition, the common short code #0 is assigned to the control channel of each of the cells.
FIG. 2 is a timing chart for explaining the conventional two-step high speed initial synchronization method, and an example of a signal of a control channel received at a mobile station. The control channel received from each of the base stations contains certain symbols, which have been spread only by the short code #0 every long-code period. Signals received from BS.sub.k-2, BS.sub.k-1, and BS.sub.k are shown in the drawing and the certain symbols are shown as shaded portions of the signals. This short code #0 is assigned commonly to all of the base stations, and this is achieved by not spreading the signals with the long codes over a certain period. The other symbol positions of the control channels are doubly spread using the long code #i, which differs from one base station to another, and the short code #0. In this way, the control channels transmitted from each of the base stations BS.sub.k-2 -BS.sub.k are asynchronously multiplexed and are received at the mobile station.
In the first stage, the mobile station finds a correlation between the base band received signal and the short code #0 using a matched filter. The peak of the correlation is detected at the temporal position corresponding to the reception timing of the symbol which is spread with the short code #0 of the control channel of the base stations. The timing corresponding to the maximum electric power is detected after these peak electric powers have been detected over the R-period of the long code. The detected timing, the code timing T, is determined to be the long code synchronization timing transmitted from the base station of the new cell in which a mobile station is located.
In the second stage, in order to identify the base station BS.sub.k, the mobile station 61 identifies the long code #k that is used to spread the control channel, the long code timing T of which has been detected. In order to accomplish this, the long code #k is sequentially selected out of the long codes #1-#n of the system in the initial cell search. A replica code is synthesized from the selected long code #k and the short code #0, and correlation is detected by a correlator using the long code synchronization timing obtained in the first stage.
The correlation detection is continued over the range of long codes #1-#n until a long code causes the correlation detection value to exceed the threshold value. The long code that has exceeded the threshold value is judged to be the long code #k of the new base station, and the cell search is completed.
When searching the neighboring cell for the hand over, similarly, a replica code is synthesized from the short code #0 and the long code #k. The long code #k is sequentially selected from the long codes of neighboring cells transmitted by the current base station to the mobile station. Correlation is detected using the long code synchronization timing.
As seen in the above, cell search can be performed at a high speed by performing the timing synchronization and the identification of the long code separately. According to the standard inter base station asynchronous cellular systems, correlation detection needs to be performed approximately (the number of spread codes x the number of phases of the spread codes) times to search a cell.
On the other hand, according to this method, it suffices to perform correlation detection (the number of spread codes+the number of phases of the spread codes) times. Therefore, in comparison with the standard inter base station asynchronous cellular systems, a cell can be searched at a high speed. However, a problem still remains when the correlation detection is performed using a correlator, as is done in the prior art. The length of time required for detecting every correlation still remains long. Hence, it is desired that the initial synchronization be performed at a higher speed.